A multi-layer printed circuit board is conventionally manufactured by first preparing a copper-plated laminate for the inner layer by processing a copper foil on the copper-plated laminate to form a printed circuit; then providing the above copper foil with a surface roughening treatment (generally comprising degreasing, followed by a soft etching process as exemplified by treatment with ammonium persulfate, sodium persulfate, cupric chloride, sulfuric acid-hydrogen peroxide system and the like, as well as an activating treatment); subsequently building an acicular film of copper oxide or cuprous oxide on top of the foil by a process such as blackening or browning; and bonding a copper-plated laminate for the outer layer or copper foils in multiple layers with a material impregnated with a thermosetting resin (i.e. a "prepreg") to fabricate a multi-layer laminated board having a high adhesion strength.
Since an electric continuity has to be established to each layer of the multi-layer laminated board manufactured in the above process, a through-hole plating on holes drilled through the board is required. However, the conventional method has had a drawback where penetration of the acid solution used in the catalyst treatment process for plating through-holes or penetration of the plating solution in the electroless copper plating process tends to dissolve the film made up from copper oxide or cuprous oxide, thereby causing a phenomenon called "pink ring" (i.e. "haloing").
On the other hand, there has been an alternative method of manufacturing a multi-layer printed circuit board whereby a printed circuit is formed on a copper-plated laminate using a copper foil that is pre-processed by surface roughening to eliminate the need for surface roughening as well as the oxide film forming processes required in the method described above. This method, however, has shortcomings such as inferior pattern resolution for the printed etching resist or the etching resist for ultra-violet exposure, which are associated with the surface roughness on the copper foil.
In recent years, methods have been developed to correct the above mentioned shortcomings on copper-plated laminates and to enhance the adhesion strength by utilizing certain particular types of electroless copper plating processing (Japanese Patent Application Laid-Open Nos. 15980/1986, 15981/1986, and 41775/1986).
The above electroless copper plating process, however, in practice requires formalin as an essential reducing agent and presents problems such as (a) abnormal copper deposition on the substrate resin, (b) unstable plating bath, or (c) an obnoxious odor as well as a potential carcinogenicity.
The present inventors have recently developed a method for forming a uniform and acicular copper film with excellent adhesion strength using electroless copper plating (Japanese Patent Application Laid-Open No. 116176/1992). While this technology enables manufacturing of a copper-plated laminate having a high adhesion strength and without the aforementioned shortcomings, surfactants that can be used in the process are limited and the benefit of the invention has not always been available.
Accordingly, it has been necessary to improve the above technology to present a technology which is able to provide better adhesion strength and is more versatile.